1. Field of the Invention
The present invention relates to an image forming apparatus utilizing an electrophotographic process or an electrostatic recording process, for example a printer such as a laser beam printer or an LED printer, or a digital copying apparatus, and more particularly to an improvement in an image quality at a fixation step thereof.
2. Related Background Art
In a prior image forming apparatus utilizing an electrophotographic process such as a laser beam printer, in case of receiving a print command, an encoded character and image information from an external information processing apparatus such as a computer and converting the code information into image information in a formatter or the like, an image having density information such as a photograph is processed by a known image processing such as a dither matrix process or an error diffusion process and binarization thereby converted into binary image information.
In the following, a prior electrophotographic engine will be explained with reference to FIG. 8.
The electrophotographic engine is provided, around a photosensitive drum (photosensitive member) 201 and along a rotating direction thereof, with a primary charger 202 for charging the photosensitive drum 202, an exposure means 203 for exposing the photosensitive drum 201 thereby forming an electrostatic latent image, a developing apparatus 204 for depositing a toner (developer) onto the electrostatic latent image thereby forming a toner image, a transfer roller (transfer apparatus) 205 for transferring the toner image on the photosensitive drum 201 onto a recording material P, and a cleaning apparatus 207 for eliminating the residual toner. The recording material P for receiving the transfer of the toner image is fed and conveyed from an unillustrated sheet cassette, and is supplied to the photosensitive drum 201. The recording material P supplied to the photosensitive drum 201 receives the transfer of the toner image by the transfer roller 205, then conveyed to a heat fixing apparatus 206 and, after a fixation of the toner image therein, is discharged to the exterior of the apparatus.
As the heat fixing apparatus 206, there is commonly employed an apparatus of heat roller type or an apparatus of film heating type. In particular, there is proposed a method of not supplying an electric power to the heat fixing apparatus in a standby state thereby minimizing the electric power consumption, more specifically a heat fixing method by a film heating method in which a film for fixing the toner image on the recording material is made present between a heater and a pressure roller. For such method, reference is to be made to following patent references 1 to 4:                Patent reference 1: Japanese Patent Application Laid-open No. S63-313182;        Patent reference 2: Japanese Patent Application Laid-open No. H02-157878;        Patent reference 3: Japanese Patent Application Laid-open No. H04-44075; and        Patent reference 4: Japanese Patent Application Laid-open No. H04-204980.        
FIG. 9 schematically shows a configuration of a heat fixing apparatus such film heating method. Referring to FIG. 9, there are provided a heating member (hereinafter referred to as heater) 211 fixed to a stay holder (support member) 212, and an elastic pressure roller 210 pressed to the heater 211 across a heat resistant thin film (hereinafter referred to as fixing film) 213 to form a nip portion (fixing nip portion) N of a predetermined nip width. The heater 211 is heated and controlled at a predetermined temperature by a current supply. The fixing film 213 is a cylindrical or endless web-shaped member or a rolled web-shaped member, conveyed in a direction of an arrow a at the fixing nip N in contact with and sliding on the heater 211, by a rotary power of unillustrated drive means or the pressure roller 220.
In a state where the heater 211 is heated and controlled at a predetermined temperature and the fixing film 213 is moved in the arrowed direction, when the recording material P, bearing an unfixed toner image t and constituting a material to be heated, is introduced between the fixing film 213 and the pressure roller 220 in the fixing nip N, the recording material P is in close contact with the surface of the fixing film 213 and is conveyed therewith through the fixing nip N. In such fixing nip N, the recording material P and the toner image t are heated by the heater 211 through the fixing film 213 whereby the toner image t is heat fixed onto the recording material P. A portion of the recording material after passing the fixing nip N is peeled off from the surface of the fixing film 213 and conveyed.
The heater 211 serving as the heating member is generally constituted of a ceramic heater. For example, it is prepared by forming, on a surface (opposed to the fixing film 213) of a ceramic substrate 211a of an electrical insulating property, a high thermal conductivity and a low heat capacity such as alumina, a heat generating resistance layer 211b such as of silver palladium (Ag/Pd) or Ta2N for example by screen printing along a longitudinal direction the substrate (perpendicular to the plane of the drawing), and covering the surface of such heat generating resistance layer with a thin glass protective layer 211c. In such ceramic heater 211, the heat generating resistance layer 211b generates heat by a current supply thereto, whereby the entire heater including the ceramic substrate 211a and the glass protective layer 211c shows a rapid temperature elevation. Such temperature elevation of the heater 211 is detected by a temperature sensor 214 provided behind the heater and fed back to an unillustrated power supply controller. The power supply controller controls the power supply to the heat generating resistance layer 211b in such a manner that the heater temperature detected by the temperature sensor 214 is maintained at a predetermined substantially constant temperature (fixing temperature). Thus the heater 211 is heated and controlled at the predetermined fixing temperature.
The fixing film 213 is made as thin as 20 to 70 μm in order to efficiently transmit the heat of the heater 211 to the recording material P, which is a member to be heated, at the fixing nip N. The fixing film 213 has a three-layered structure of a base film layer, a primer layer and a releasing layer, in which the base film layer is at the side of the heater and the releasing layer is at the side of the pressure roller. The base film layer is formed by polyimide, polyamidimide or PEEK, having a higher insulting property than in the protective glass layer 211c of the heater 211, and being highly heat resistant and having a high elasticity. Also the base film layer maintains the mechanical strength such as tensile strength of the entire fixing film 213. The primer layer is formed by a thin layer of about 2 to 6 μm. The releasing layer is provided for preventing toner offsetting to the fixing film 213, and is formed by coating a fluorinated resin such as PFA, PTFE or FEP in a thickness of about 10 μm.
A stay holder 212 is formed for example of a heat resistant plastic member, and supports the heater 211 and also serves as a conveying guide for the fixing film 213.
In a heat fixing apparatus of the film heating type utilizing such thin fixing film 213, because of a high rigidity of the ceramic heater 211 serving as the heating member, the pressure roller 220 having an elastic layer 222 becomes flat along the flat lower surface of the heater 211 to which the roller is pressed, thereby forming a fixing nip N of a predetermined width and the heating is achieved only in the fixing nip N to attain a quick-start heat fixing.
The above-described prior image forming apparatus is known to cause various drawbacks in the image quality at the image fixation.
For example, there are known an offset phenomenon in which, at the heat fixing of an unfixed toner image on the recording material, a part of the toner is not fixed but is deposited on the fixing film and is transferred to the recording material in a next turn of the fixing film, and a fixation tailing phenomenon in which an unfixed toner image is scattered, by vapor ejected from the recording material, in a direction opposite to the conveying direction of the recording material.
As it is known that these phenomena can be alleviated by applying a bias voltage of a polarity same as that of the toner to the fixing roller or the fixing film thereby forming an electric field, in the fixing nip, in a direction of pressing the toner toward the recording material, there is adopted a structure of applying a bias voltage to the fixing roller or the fixing film by high-voltage output means. Also as the high-voltage output means for applying the bias voltage to the fixing roller or the fixing film, a high voltage source used in image forming steps such as charging or development is often utilized, thereby achieving a cost reduction and a compactization of the apparatus.
However, a recent higher process speed in the image forming apparatus tends to aggravate the fixation tailing smears (bleeding) and the offsetting, thus requiring a higher bias voltage as a fixing bias. However, in case the fixing bias voltage is supplied from the high voltage source used for other image forming steps such as charging or development, the fixing bias voltage is determined by an output voltage of such high voltage source so that an even higher bias voltage is not available. More specifically, a charging DC bias voltage and a developing DC bias voltage are determined by certain conditions such as an image density, a line width and a fog level, and the charging DC bias voltage is generally selected at −600 to −700 V while the developing DC bias voltage is generally selected at −400 to −500 V, so that the fixing bias voltage of a larger value cannot be utilized.
In order to avoid such drawback, there can be conceived a method of independently providing a high voltage source for the fixing bias or a method of providing a power source capable of outputting a voltage necessary for the fixing bias and dividing such output voltage for example by voltage-dividing resistors to necessary values for the charging bias or the developing bias, but the former method is associated with drawbacks of an increased dimension and an increased cost of the apparatus, while the latter method has a drawback that the voltage drop by the voltage-dividing resistors varies depending on a load to generate an aberration in the bias voltage, thereby resulting in an uneven density in a halftone image such as a graphic image.